1. Field of the Invention
The present invention relates to a method of coating a sensitizing solution onto a metal plate used for manufacturing shadow masks for color cathode ray tubes, and a coating apparatus.
2. Description of the Related Art
Conventionally, a coating apparatus has been known which coats a sensitizing solution onto a metal plate for a shadow mask in the process of manufacturing that shadow mask for color cathode ray tubes. The coating apparatus comprises, in a sequential order, a supply machine having a roll of elongated metal sheet, i.e., iron sheet, for drawing the sheet from the roll, seam welder, back tension device for imparting a back tension to the running iron sheet, washing tank, drying furnace, coating machine, drying furnace, drive device for running the iron sheet, and winding machine for winding-up the iron sheet. The seam welder is used to seam the ends of iron sheets in the case where, when one iron sheet is exhausted on the supply machine, another iron sheet is fed out of another supply machine.
The coating machine includes a top-opened vessel for storing a sensitizing solution. In the bottom wall of the vessel is formed a slit through which the iron sheet passes from the underside of the vessel to the upper-side thereof. Wipers are provided around the slit and slidably and intimately contact with the iron sheet in a liquid-tight fashion, preventing the solution from leaking downward through the slit.
The sensitizing solution coating process using the conventional apparatus will be explained below.
The iron sheet drawn from the supply machine passes through the lower portion of the seam welder and the back tension device to the washing tank where dust and smudges are washed away from the surfaces of the iron sheet. Then the sheet, after drying up by the drying furnace, is run vertically past the coating machine.
Upon the passage of the iron sheet through the coating machine, it enters the vessel, via the slit, from below and is moved in the sensitizing solution in the vessel and lifted up from the top-opened end of the vessel with the solution coated on the whole surfaces of the iron sheet.
The solution-coated iron sheet passes through the drying furnace where the solution dries up by heat on the surfaces of the iron sheet with water evaporated, thus forming resist layers on the surfaces of the iron sheet. The resultant iron sheet is run past the iron sheet drive device to the winding machine where it is wound thereon.
The thickness of the resist layer in the dried state is determined depending upon the concentration and viscosity of the solution stored in the vessel, lift-up speed of the iron sheet being run through the vessel, and heating energy in the drying furnace. That is, the enhanced concentration and viscosity of the sensitizing solution, increased lift-up speed of the iron sheet and increased heating energy at the drying furnace result in an increase in the thickness of the resist layer.
If the concentration of the solution is enhanced so as to increase the thickness of the resist layer, the viscosity of the solution is also increased. However, too high a viscosity results in generating air bubbles in the sensitizing solution and hence in a greater variation in the thickness of the resist layer
Further, if the lift-up speed of the iron sheet is increased, the coated iron sheet does not sufficiently dry up unless the length of the drying furnace is increased. If the heating energy at the drying furnace is too great, only the surface region of the coated layer on the iron sheet quickly dries up and becomes heavier, but that region of the coated layer which is close to the surface of the iron sheet remains moist, causing a drip or run of the coated solution. Thus, the thickness of the coated layer becomes unstable This is partly because the coated iron sheet is dried by the drying furnace in a vertical state. The iron sheet vertically lifted up from within the coating machine has all surfaces thereof coated with the solution and, therefore, has to dry up in the vertical state.
In this way, the conventional method includes a very unstable coating step.
As set out above, since the concentration and viscosity of the sensitizing solution cannot be made too high and the iron sheet has its whole surfaces pass through the solution, the resist layer thus formed is made thicker at each side edge portion called "fat edge" portion. That is, with t1 representing the thickness of the resist layer at a central area on one surface of the iron sheet; t2, the thickness of the end edge portions; and t3, the thickness of the resist layer at a central area on the other surface of the iron sheet, t1=t3 but, at t2=a.times.t1, a=about 1.3 to 1.4.
The "fat edge" phenomenon is not desirable because the iron sheet is irregularly wound up on rollers, etc., at the subsequent step or steps. This is due to the fact that, upon being wound around a winding cylinder, the iron sheet suffers a load only at its side edges and hence has its central area create a gap between its adjacent turns of the iron sheet coil on the cylinder
Further, the entire iron sheet passes through the sensitizing solution and is coated with the solution also at each side edge. The solution deposited on the respective side edge of the iron sheet is originally required, but, once being dried, the dried portion is liable to be peeled from the dried surface of the iron sheet, sometimes causing a fall of dust in the subsequent step or steps. In a later exposure step in particular, an original sensitizing plate is soiled by such dust and hence the deposition of the solution on each side edge of the iron sheet should initially be avoided.
The iron sheet having the resultant resist layer is transferred to an etching step through an exposure step. In the etching step following the exposure step, the iron sheet is etched after an etching solution is sprayed on each resist layer, whereby electron beam passage holes are formed at those positions corresponding to those unexposed resist layer portions of the iron sheet. By so doing, larger holes are provided in one surface side and corresponding smaller holes are provided in the other surface side of the iron sheet to provide communication between each larger hole and the corresponding smaller hole.
In the recent etching step, in order to reduce the etching time, a higher spray pressure has been employed upon the spraying or jetting of the etching solution on the resist layer. This tendency is prominent when the larger holes in particular are formed in the iron sheet. However, the use of too high a spray pressure produces a defect in the resist layer, failing to open electron beam passage holes, as designed, in the iron sheet. This problem can be solved by making the resist layer on the larger hole side thicker than that on the smaller hole side of the iron sheet. In the conventional coating method as set out above, however, it is not possible to vary the coating layer thickness at both surface sides of the iron sheet, that is, the same layer thickness is formed at each surface side of the iron sheet. Further, it is not possible to form a considerably thick resist layer on the iron sheet surface because the sensitizing solution is not made higher in concentration and viscosity.